1. Field of the Invention
This invention relates to an error control circuit for use in digital communication systems.
2. Description of the Prior Art
Digital communication systems of the continuous loop variety are now being looked to for use in situations where the various stations connected to the loop share a close physical proximity to each other. In some applications such systems are called closed-ring time division multiplex systems and may use pulse code modulation. Such a system is shown in U.S. Pat. No. 4,049,921 issued Sept. 20, 1977 to Edward Zwack, which patent is hereby incorporated by reference herein as if reproduced in full.
The Zwack patent discloses a closed loop communication system which allows conferencing between several stations. The stations are connected to the digital loop by an interface circuit which operates to add the instantaneous digital or analog signal onto the conference bus and to remove from the bus that portion of the combined signal which had been placed on the bus via the same interface circuit during the last cycle. The system operates with a delay circuit where the signal from the station is delayed one full cycle at the interface circuit and compared to the incoming signal for the purpose of subtracting from the incoming signal the portion of that combined signal which had been placed on the loop in the previous cycle. A problem exists with such systems in that, while the interface circuits operate to remove signals from the loop which were provided from the associated station nothing protects the loop from signals which have been erroneously placed thereon. The problem stems from the fact that the loop relies upon each interface circuit to remove from the loop those signals placed on the loop by that interface. However, those signals which, for one reason or another, enter the loop because of an error condition remain on the loop and, unless removed, are equivalent to a DC component. A DC component in the signal gives an undesirable shift of the signal toward its digital limits. A signal shifted beyond its limit produces overflow, an unacceptable situation analogous to saturation of a linear analog system.
In copending application of Baugh there is disclosed an arrangement for controlling errors in such a loop system. The Baugh disclosure is directed to a linear analog approach to the problem. Thus one object of my invention is to use a technique more in keeping with the digital nature of the communication signals. This results in a time and cost efficient alternative to loop error correction and also overcomes the "deadband" decay problem caused by finite resolution of digital signals.